Distinguishing features: Nannoliths with rhombic and tri-radiate morphologies, and non-birefringent images in XPL. Incorporating the genera Rhomboaster and Tribrachiatus.

Morphology remarks: Crystallography: the c-axis is parallel to rotational axis of the nannolith and so in plan view they show no birefringence, they are, however, often tilted and so show moderate birefringence. IN side view they show high birefringence.

Geological Range: Last occurrence (top): in upper part of Ypresian Stage (67% up, 50.5Ma, in Ypresian stage). Data source: Total of range of species in this databaseFirst occurrence (base): within Ypresian Stage (47.84-55.96Ma, base in Ypresian stage). Data source: Total of range of species in this database

Bybell, L. & Self-Trail, J. (1997). Late Paleocene and Early Eocene calcareous nannofossils from three boreholes in an onshore-offshore transect from New Jersey to the Atlantic continental rise. Proceedings of the Ocean Drilling Program. Scientific Results, 150: 91-110. GS